1. Field of the Invention
The present invention relates to metal protection against corrosion, and particularly to compositions intended for a diffusion coating of ferrous metals. The invention can be used most effectively for corrosion protection of parts and assemblies of the equipment used in chemical industries for the production of soda and soda products, magnesium and barium chlorides, and sodium sulfates.
2. Description of the Prior Art
One of the main problems facing manufacturers of chemical apparatus is the provision of high corrosion resistance in the apparatus parts at minimum cost required. Attempts to apply prior art compositions for the formation of protective coating on the surface of apparatus parts, which operate in highly concentrated salt solutions and chloride-containing media at temperatures of 60.degree. to 100.degree. C., have demonstrated that in many cases these measures either do not compensate for an increase in apparatus service life (when rare elements in compositions and complicated methods of coating are used), or do not provide for any significant increase in corrosion resistance (when cheap compositions and comparatively simple procedure of coating are used). This points to the fact that this problem has not yet been solved satisfactorily enough.
Experience has proved that best results are achieved by a diffusion saturation of ferrous metals with titanium. Known in the art are various compositions for a diffusion coating, which compositions contain titanium or its compounds.
Some attempts are known to use titanium powder for the diffusion coating of ferrous metals (as disclosed in Japanese Patent Publication No. 49-3899). Metallic titanium is applied to the structural component by any convential method e.g. vacuum spraying. The coating obtained is a solid solution of titanium in iron and does not contain the carbide phase. For this reason its corrosion resistance is low, and structural components made of ferrous metals and protected with such a coating cannot resist long-term contact with aggressive liquids.
There is known a composition for the diffusion coating of ferrous metals which composition containing titanium compound (titanium carbide, nitride or carbonitride), alkaline halohemide (chlorine or fluorine). The composition is applied to a workpiece being coated in an isothermal chamber in an atmosphere of inert gas, hydrocarbon or nitrogenated gas at temperatures of 800.degree. to 1150.degree. C. as disclosed in French Patent No. 2181512. This composition contains titanium carbides which provide for a better resistance of the coating in comparison with that as above described.
However, titanium nitride or carbonitride contained in the coating composition causes a considerable increase in hardness (up to 1800 kg/mm.sup.2 Vickers hardness), and a corresponding decrease in plasticity and corrosion resistance. If the coating consists of titanium carbide only, the diffusion layer is of inadequate continuity and does not resist aggressive media. Besides, the method of applying such composition is not economically feasable because of high cost of the inert gas used in the process.
Known in the art is a composition for the diffusion coating of ferrous metals and containing a particulate mixture of titanium, alumina and ammonium halide in the following ratio (% wt):
titanium: 75 PA0 alumina: 24 PA0 ammonium chloride: 1 PA0 titanium: 70.0 to 82.0 PA0 alumina: 14.5 to 20.0 PA0 ammonium halide: 2.0 to 5.0 PA0 graphite: 1.0 to 2.0
This composition is a powdered mixture which is heated during the coating process together with the workpieces in a hermetically sealed container at 1050.degree. C. and is held at this temperature for 3 hours (see A. N. Minkevich, Khimiko-termicheskaya obrabotka metallov i splavov, Moskva, "Mashinostroyenie" 1965, p.294).
This composition makes it possible to obtain a coating of adequate continuity in the diffusion layer due to the presence of alumina inert admixture and provides for formation of titanium carbide, titanium compounds with iron, and titanium nitrides in the diffusion layer.
As can be seen from the referenced publication, diffusion layer application is a comparatively simple procedure and does not involve the use of such an expensive auxiliary raw material as inert gas. However, corrosion resistance of such diffusion layer is adversely affected by titanium nitrides which are formed during interaction of air, nitrogen, and ammonia with titanium while being heated. Titanium nitrides significantly increase brittleness of the diffusion layer and are comparatively easily solved in such chloride-containing media and highly concentrated salt solutions. Amount of nitrides in the diffusion layer could be decreased by a protective atmosphere of inert gas in the process of diffusion layer application. However, this would bring about considerable additional expenses.